This invention relates generally to microscope systems of the type which employ subatomic particles in the imaging process, and more particularly, to transmission and re-emission positron (e.sup.+) microscope systems wherein low energy, or slow, positron beams are employed.
Notwithstanding that during the past ten years it has become possible as a result of advances in technology to produce low energy (on the order of 1 eV) positron beams from an initially high energy positron source, typically with an intensity on the order of 10.sup.7 e.sup.+ /sec., it is generally believed by persons skilled in this art that the development of an effective positron microscope system is not feasible at the present due to the low intensity and current density of available positron beams. This is entirely contradistinct from the ease with which electron (e.sup.-) beams are generated having sufficiently high flux to permit production of images visible to the eye. Such e.sup.- beams, which are easily produced, illustratively from a heated tungsten filament, are of high quality and have magnitudes of current on the order of tens of microamperes (10.sup.15 e.sup.- /sec.).
In addition to having low current densities, current slow positron beams are reduced in their output flux by a factor of at least 10.sup.3 during improvement of beam quality to meet the requirements necessary to produce a good image. This factor results from the following:
1. Between the source and the sample being investigated a set of electron focussing lenses are required so as to produce a highly parallel beam which is extremely small. Such focussing results in a diminution in beam intensity by a factor of 10.
2. After the beam interacts with the target, a further set of electron lenses selects a very narrow angular cone of the emitted beam so as to produce a high contrast, high resolution image. Such angular selection results in an additional factor of 10.sup.2 loss in rate.
3. Finally, the beam is magnified and projected onto a detector. As a result of the limits on the size of the detector, this magnification (M) process can result in further losses which increase as a function of M.sup.2. It is therefore evident, that for the electron source, approximately 10.sup.12 e.sup.- /sec. can be utilized in image formation while in a positron environment, at best, only 10.sup.4 e.sup.+ /sec. are available.
Under the best possible conditions, the human eye can detect an image produced by beam current densities as low as 10.sup.8 e.sup.- /cm.sup.2 -sec. incident on a phosphor screen. With the use of currently available image intensifiers, plus computer-based signal averaging, it is possible to detect images at densities as low, or lower, than 10.sup.6 e.sup.- /cm.sup.2 -sec. Under the lowest possible useful microscope magnification, the positron beam discussed above will have a current density of 10.sup.5 e.sup.+ /cm.sup.2 -sec. This is approximately one order of magnitude below what is normally used in high resolution, low dose, electron microscopy, and is one of the primary reasons why positron microscopy has not been considered to be feasible.
The significance of a positron microscope system in the art is made evident from an understanding of the manner in which positrons interact with matter. When positrons are injected into matter, a number of interactions occur between the injected particle and the medium into which it is injected. A positron may undergo one or more scatterings, it may be backscattered out of the medium, or it may cause ejection of a secondary electron. If the medium is sufficiently thin, and if the positron has sufficient energy, it may be transmitted right through the medium.
In the transmission mode of operation, no significant new physics is expected to occur at low magnifications in the operation of a transmission positron microscope. This is because the fundamental interactions responsible for removing positrons from a transmission positron microscope beam are essentially identical to those for removing electrons from a transmission electron microscope beam. On the other hand, diffraction patterns obtained from thin samples could be studied in a manner analogous to diffraction studies using transmission electron microscopes. Such diffraction patterns will be different from corresponding electron diffraction patterns, particularly at energies below 10 keV.
It should also be possible to exploit the fact that the positron beam of the present invention is spin polarized with polarizations as high as P=0.7 being possible, albeit at a factor of 5 decrease in beam intensity. This should make it possible to perform Polarized Low Energy Positron Diffraction (PLEPD) in the transmission mode. The ensemble of high energy positrons emitted from the nuclear beta decay of the radioactive isotopes most commonly used to produce slow positron beams are naturally spin polarized as a consequence of the weak interaction that created them. The spin polarization of the high energy positrons is retained during their slowing down in the slow positron moderator. As a consequence, the slow positrons emitted from the moderator are also spin polarized. Acceleration and focusing of the slow positrons allows the formation of a spin polarized beam. The degree of spin polarization of the slow positron beam can be controlled by placing absorbers of a low atomic number between the source of high energy positrons and the moderator. The absorber acts on the principle that the lower energy part of the radioactive source spectrum (initially some tens of keV), which, as a consequence of the weak interaction, has a lower degree of spin polarization, stops in the absorber, rather than the moderator. Only the higher spin polarized, initially high energy positron of the source spectrum passes through the absorber to the moderator, and therefore the resultant slow positrons produced by the moderator have a higher degree of spin polarization. The direction of spin polarization of the resultant positron beam can be controlled by the suitable application of crossed electrostatic and magnetic fields. Polarized positron microscopy, as a complement to the recently developed polarized electron microscope, should yield information on the exchange interaction and other spin-polarized phenomena.
Although the foregoing interactions occur for both, positrons and electrons, the fact that positrons are antimatter cause them to have certain characteristics which cause them to undergo types of interactions which are not shared by electrons. For example, a positron may combine with an electron, resulting in annihilation of the particles and emission of two gamma-rays in substantially opposite directions, each having an energy (E=mc.sup.2) of approximately 511,000 eV. Alternatively, a positron may capture an electron to form the hydrogen-like positron-electron bound state called positronium (Ps). The Ps atom may annihilate in the medium into two or three gamma rays after a characteristic Ps lifetime, typically on the order of 1 to 3 nanoseconds, depending on the medium. Alternatively, the Ps may escape from the medium and live in a vacuum with a 140 nanosecond lifetime. In addition to the foregoing, the positron might stop in the medium and travel slowly (diffuse) to the surface where it can be ejected from the medium by electric fields which exist at most surfaces in a vacuum. This surface electric field, which is also known as the work function, typically pulls electrons back into the medium, but can operate to repel positrons out of the medium.
The phenomenon of expelling slow positrons from the medium is known as "slow positron emission," and forms the basis of production of slow positron beams. In certain regards, this phenomenon is similar to the process of electron field emission.
In addition to the foregoing, the positron injected into a medium may be trapped in a defect, which is the absence of a positively charged atom. The positron will live in the defect for a lifetime which is to an extent determined by the size, charge, and other properties of the defect, after which the positron is annihilated. Thus, a positron microscope system can be expected to produce images resulting from at least the four phenomena: annihilation, positronium formation, slow positron emission, and defect trapping, such images not being achievable with electron microscopy.
The foregoing notwithstanding, there do exist significant similarities between electrons and positrons, and in the manners in which microscopy employing these particles can be conducted. Referring for the moment to electron microscope systems, the electron microscope can take different forms, illustratively the transmission electron microscope (TEM) and the reflection electron microscope (REM). In both such types, a scanning process may be applied to achieve imaging of relatively large areas at high magnification.
The TEM operates by transmitting a high energy (20 keV to 1 MeV) electron beam having a small diameter on the order of 10.sup.-5 meter through a thin slice of the material to be studied. During transmission of the beam through the sample, different parts of the beam are strongly scattered out of the beam, or blocked. The degree of scattering is dependent on variations in the composition of the target. After transmission, the initially uniformly distributed beam is characterized by regions of low intensity where the sample has preferentially eliminated electrons from the beam, and regions of high intensity where the sample and beam had little interaction. Thus, an image of the sample is contained in the information in the intensity distribution in the beam. The relative difference in intensity between the regions of high and low intensity is called the contrast of the image. At this point, just after transmission through the sample, the beam and its image information are still contained in the original 10.sup.-5 meter beam diameter, and the image size is identical to the size of the different features in the sample which produced the regions of high and low intensity. This narrow beam propagates to the vicinity of a series of powerful electron lenses, which are usually magnetic fields, and which are applied to increase the beam diameter from 10.sup.-5 meters up to several meters in diameter without distortion of the image information contained in the beam. This can result in a magnification factor of up to 10.sup.6. A large fraction of the outer edge of the beam is therefore lost in the magnification process. The magnified high energy electron beam is then allowed to strike a phosphor screen where the electron kinetic energy is converted into light, producing an optical image, with regions of high intensity corresponding to the absence of a given feature. At this point, a feature of 1 .ANG. diameter (10.sup.-8 cm) in the sample will appear on the phosphor screen as a dark area of 0.1 mm in diameter, a size which can be seen by the human eye.
The ability to distinguish small features on the image is called the "resolution" of the electron microscope. For the 1 .ANG. feature discussed above, the resolving power of the microscope is 1 .ANG.. In principle, features of any size can be resolved from each other with high enough magnification and beam current density. In practice, however, quantum mechanical effects limit the resolving power of the electron microscope to distinguishing features of about 1 .ANG. from each other.
Another phenomenon which appears in the TEM is diffraction. This effect is primarily quantum mechanical in nature and arises from the wave nature of the particles involved. In certain regards, the diffraction effect is qualitatively similar to the wave patterns produced when two waves intersect on a pond. The wave pattern of the incident electron will interact with the different wave patterns of electrons in the sample in a way which produces highly regular patterns of high transmission and low transmission. Each different type of molecule has its own unique diffraction pattern which identifies it like a fingerprint. Thus, the diffraction patterns can be used to identify the composition of a given sample. The diffraction patterns are also sensitive to changes in the chemical binding of one molecule to another, to the orientation of any crystal planes which may be in the sample, and to some types of defects in the sample.
The Reflection Electron Microscope (REM) has magnifying optics as does the TEM, but these optics magnify images resulting from electrons which scatter backwards from the initial beam direction. These electrons are produced primarily from two processes, and include: (1) elastically backscattered electrons which retain their high energy, and (2) secondary electrons which are emitted with about 30 volts of energy. As a result of their low energy, detection techniques which are different from those used in TEM are frequently used to form an image from the secondary electrons.
The images formed from the two types of electrons emitted will highlight different features of the sample, because the basic interactions involved in their production are different. Backscattered electrons are produced primarily from the nuclei of the atoms, whereas secondary electrons are produced by interactions with the electrons in the medium. Thus, complementary features can be compared directly.
The basic advantage of the reflection mode of electron microscopy is that it obviates the need to make thin slices of the sample to form an image. A wider range of targets, including targets which would be destroyed by slicing, can be studied using the REM. Such targets include, for example, integrated circuit chips.
The formation of images from secondary electrons becomes particularly powerful when combined with a scanning technique. No essential difference should exist between the deflection plate design of a scanning electron microscope, and that of a scanning positron microscope. In both cases, the deflection plates would consist of two independent parallel pairs rotated at 90.degree. with respect to one another. One pair controls the x position of the beam, and the other pair controls the y position of the beam. The x, y position of the beam is controlled by application of varying electric fields to the pairs of the plates. The plates are located as the last element in the electron optical system prior to the beam hitting the target.
In this mode of operation an extremely small beam, having a size typically on the order of 10.sup.-8 meters in diameter, is swept along the surface of the target by means of electric deflection plates. The secondary electron current is detected as the beam sweeps the target and an image is formed from the variation in current as a function of position. Using the scanning technique allows the use of low energy electron beams (less than 1000 volts), which reduces the amount of damage to the sample, and also reduces the time required to examine each specimen.
It is, therefore, an object of this invention to provide a positron microscope system.
It is another object of this invention to provide a positron microscope system which utilizes slow positron beams.
It is also an object of this invention to provide a positron microscope system which is simple and inexpensive.
It is a further object of this invention to provide a positron microscope system which can produce images of selected types of positron-electron interactions.
It is additionally an object of this invention to provide a positron microscope system wherein imaging can be achieved in a backscattering, or reflection, mode of operation.
It is yet a further object of this invention to provide a positron microscope system wherein imaging can be achieved in a transmission mode of operation.
It is yet an additional object of this invention to provide a modified electron optical lens system for use with a moderated positron beam.
It is still another object of this invention to provide a positron microscope system wherein imaging can be achieved using low beam current densities.
It is also a further object of this invention to provide a positron microscope system which utilizes the phenomenon of slow positron re-emission to produce an image.
It is additionally another object of this invention to provide a positron microscope system which utilizes the phenomenon of positron annihilation to produce an image.
A further object of this invention is to provide a positron microscope system which utilizes the phenomenon of positronium formation to produce an image.
An additional object of this invention is to provide a positron microscope system which utilizes the phenomenon of defect trapping to produce an image.
Another object of this invention is to provide a positron microscope system which can produce a spatial image.
A yet further object of this invention is to provide a system which can be used to study electron momentum and distribution of electron momenta.
It is also an additional object of this invention to provide a system which can be used to generate a spatial image corresponding to the distribution of electron momenta.
A still further object of the invention is to provide a positron microscope system which utilizes positronium atoms to generate an image.
An additional object of the invention is to provide a positron microscope system which utilizes gamma rays resulting from positron annihilation to generate an image.
Yet another object of the invention is to provide a system which utilizes spin polarized positrons to facilitate diffraction studies.
Also, it is an object of this invention to provide a positron microscope system which can generate correlated images using transmission and reflection modes of operation simultaneously.
Still another object of this invention is to use spin polarized positrons to generate images.